Technical Field
Methods and example implementations described herein are directed to a streaming bridge design, and more specifically, to a bridge design that is operatively coupled with multiple System-on-Chip (SoC) host interfaces and Network on Chip (NoC) Layers.
Related Art
The number of components on a chip is rapidly growing due to increasing levels of integration, system complexity and shrinking transistor geometry. Complex System-on-Chips (SoCs) may involve a variety of components e.g., processor cores, DSPs, hardware accelerators, memory and I/O, while Chip Multi-Processors (CMPs) may involve a large number of homogenous processor cores, memory and I/O subsystems. In both SoC and CMP systems, the on-chip interconnect plays a role in providing high-performance communication between the various components. Due to scalability limitations of traditional buses and crossbar based interconnects, Network-on-Chip (NoC) has emerged as a paradigm to interconnect a large number of components on the chip. NoC is a global shared communication infrastructure made up of several routing nodes interconnected with each other using point-to-point physical links.
Messages are injected by the source and are routed from the source node to the destination node over multiple intermediate nodes and physical links. The destination node then ejects the message and provides the message to the destination. For the remainder of this application, the terms ‘components’, ‘blocks’, ‘hosts’ or ‘cores’ will be used interchangeably to refer to the various system components that are interconnected using a NoC. Terms ‘routers’ and ‘nodes’ will also be used interchangeably. Without loss of generalization, the system with multiple interconnected components will itself be referred to as a ‘multi-core system’.
There are several protocols by which components can connect to a network. Several industry standards are in existences such as AXI, PCI, etc. In addition, several internal protocols have been developed for communication between components. In a complex system-on-chip, there are close to a hundred components all connected to the same network by which they communicate with memory. These components have evolved through different periods of time and through different architectural and performance preferences, due to which they chose to adopt different interface protocols. Components that expect to connect to each other over a NoC must convert now their communication into a language that is understood by each intended destination.
Packets are message transport units for intercommunication between various components. A NoC provides maximum benefit to a system when requests and responses are packetized. The use of packets allows a reduction in hardware cost so that no dedicated connections are required between components. Existing connections can be time shared for packets from different sources going to different destinations. If a section of the NoC is compromised, packets can be automatically re-routed to provide a graceful degradation of the system instead of a deadlock. Hence, packetizing and bridging the chosen component protocol to a NoC provides a good interconnect solution.
Examples of routing techniques include deterministic routing, which involves choosing the same path from A to B for every packet. This form of routing is independent from the state of the network and does not load balance across path diversities, which might exist in the underlying network. However, such deterministic routing may implemented in hardware, maintains packet ordering and may be rendered free of network level deadlocks. Shortest path routing may minimize the latency as such routing reduces the number of hops from the source to the destination. For this reason, the shortest path may also be the lowest power path for communication between the two components. Dimension-order routing is a form of deterministic shortest path routing in 2-D, 2.5-D, and 3-D mesh networks. In this routing scheme, messages are routed along each coordinates in a particular sequence until the message reaches the final destination. For example in a 3-D mesh network, one may first route along the X dimension until it reaches a router whose X-coordinate is equal to the X-coordinate of the destination router. Next, the message takes a turn and is routed in along Y dimension and finally takes another turn and moves along the Z dimension until the message reaches the final destination router. Dimension ordered routing may be minimal turn and shortest path routing.
FIG. 1(a) pictorially illustrates an example of XY routing in a two dimensional mesh. More specifically, FIG. 1(a) illustrates XY routing from node ‘34’ to node ‘00’. In the example of FIG. 1(a), each component is connected to only one port of one router. A packet is first routed over the x-axis till the packet reaches node ‘04’ where the x-coordinate of the node is the same as the x-coordinate of the destination node. The packet is next routed over the y-axis until the packet reaches the destination node.
In heterogeneous mesh topology in which one or more routers or one or more links are absent, dimension order routing may not be feasible between certain source and destination nodes, and alternative paths may have to be taken. The alternative paths may not be shortest or minimum turn.
Source routing and routing using tables are other routing options used in NoC. Adaptive routing can dynamically change the path taken between two points on the network based on the state of the network. This form of routing may be complex to analyze and implement.
A NoC interconnect may contain multiple physical networks. Over each physical network, there may exist multiple virtual networks, wherein different message types are transmitted over different virtual networks. In this case, at each physical link or channel, there are multiple virtual channels; each virtual channel may have dedicated buffers at both end points. In any given clock cycle, only one virtual channel can transmit data on the physical channel.
NoC interconnects may employ wormhole routing, wherein, a large message or packet is broken into small pieces known as flits (also referred to as flow control digits). The first flit is the header flit, which holds information about this packet's route and key message level info along with payload data and sets up the routing behavior for all subsequent flits associated with the message. Optionally, one or more body flits follows the head flit, containing the remaining payload of data. The final flit is the tail flit, which in addition to containing the last payload also performs some bookkeeping to close the connection for the message. In wormhole flow control, virtual channels are often implemented.
The physical channels are time sliced into a number of independent logical channels called virtual channels (VCs). VCs provide multiple independent paths to route packets, however they are time-multiplexed on the physical channels. A virtual channel holds the state needed to coordinate the handling of the flits of a packet over a channel. At a minimum, this state identifies the output channel of the current node for the next hop of the route and the state of the virtual channel (idle, waiting for resources, or active). The virtual channel may also include pointers to the flits of the packet that are buffered on the current node and the number of flit buffers available on the next node.
The term “wormhole” plays on the way messages are transmitted over the channels: the output port at the next router can be so short that received data can be translated in the head flit before the full message arrives. This allows the router to quickly set up the route upon arrival of the head flit and then opt out from the rest of the conversation. Since a message is transmitted flit by flit, the message may occupy several flit buffers along its path at different routers, creating a worm-like image.
Based upon the traffic between various end points, and the routes and physical networks that are used for various messages, different physical channels of the NoC interconnect may experience different levels of load and congestion. The capacity of various physical channels of a NoC interconnect is determined by the width of the channel (number of physical wires) and the clock frequency at which it is operating. Various channels of the NoC may operate at different clock frequencies, and various channels may have different widths based on the bandwidth requirement at the channel. The bandwidth requirement at a channel is determined by the flows that traverse over the channel and their bandwidth values. Flows traversing over various NoC channels are affected by the routes taken by various flows. In a mesh or Taurus NoC, there may exist multiple route paths of equal length or number of hops between any pair of source and destination nodes. For example, in FIG. 1(b), in addition to the standard XY route between nodes 34 and 00, there are additional routes available, such as YX route 203 or a multi-turn route 202 that makes more than one turn from source to destination.
In a NoC with statically allocated routes for various traffic slows, the load at various channels may be controlled by intelligently selecting the routes for various flows. When a large number of traffic flows and substantial path diversity is present, routes can be chosen such that the load on all NoC channels is balanced nearly uniformly, thus avoiding a single point of bottleneck. Once routed, the NoC channel widths can be determined based on the bandwidth demands of flows on the channels. Unfortunately, channel widths cannot be arbitrarily large due to physical hardware design restrictions, such as timing or wiring congestion. There may be a limit on the maximum channel width, thereby putting a limit on the maximum bandwidth of any single NoC channel.
Additionally, wider physical channels may not help in achieving higher bandwidth if messages are short. For example, if a packet is a single flit packet with a 64-bit width, then no matter how wide a channel is, the channel will only be able to carry 64 bits per cycle of data if all packets over the channel are similar. Thus, a channel width is also limited by the message size in the NoC. Due to these limitations on the maximum NoC channel width, a channel may not have enough bandwidth in spite of balancing the routes.
To address the above bandwidth concern, multiple parallel physical NoCs may be used. Each NoC may be called a layer, thus creating a multi-layer NoC architecture. Hosts inject a message on a NoC layer; the message is then routed to the destination on the NoC layer, where it is delivered from the NoC layer to the host. Thus, each layer operates more or less independently from each other, and interactions between layers may only occur during the injection and ejection times. FIG. 2(a) illustrates a two layer NoC. Here the two NoC layers are shown adjacent to each other on the left and right, with the hosts connected to the NoC replicated in both left and right diagrams. A host is connected to two routers in this example—a router in the first layer shown as R1, and a router is the second layer shown as R2. In this example, the multi-layer NoC is different from the 3D NoC, i.e. multiple layers are on a single silicon die and are used to meet the high bandwidth demands of the communication between hosts on the same silicon die. Messages do not go from one layer to another. For purposes of clarity, the present application will utilize such a horizontal left and right illustration for multi-layer NoC to differentiate from the 3D NoCs, which are illustrated by drawing the NoCs vertically over each other.
In FIG. 2(b), a host connected to a router from each layer, R1 and R2 respectively, is illustrated. Each router is connected to other routers in its layer using directional ports 301, and is connected to the host using injection and ejection ports 302. A bridge-logic 303 may sit between the host and the two NoC layers to determine the NoC layer for an outgoing message and sends the message from host to the NoC layer, and also perform the arbitration and multiplexing between incoming messages from the two NoC layers and delivers them to the host.
In a multi-layer NoC, the number of layers needed may depend upon a number of factors such as the aggregate bandwidth requirement of all traffic flows in the system, the routes that are used by various flows, message size distribution, maximum channel width, etc. Once the number of NoC layers in NoC interconnect is determined in a design, different messages and traffic flows may be routed over different NoC layers. Additionally, one may design NoC interconnects such that different layers have different topologies in number of routers, channels and connectivity. The channels in different layers may have different widths based on the flows that traverse over the channel and their bandwidth requirements.
In a NoC interconnect, if the traffic profile is not uniform and there is a certain amount of heterogeneity (e.g., certain hosts talking to each other more frequently than the others), the interconnect performance may depend on the NoC topology and where various hosts are placed in the topology with respect to each other and to what routers they are connected to. For example, if two hosts talk to each other frequently and require higher bandwidth than other interconnects, then they should be placed next to each other. This will reduce the latency for this communication which thereby reduces the global average latency, as well as reduce the number of router nodes and links over which the higher bandwidth of this communication must be provisioned.
Moving two hosts closer together may make certain other hosts far apart since all hosts must fit into the 2D planar NoC topology without overlapping with each other. Thus, various tradeoffs must be made and the hosts must be placed after examining the pair-wise bandwidth and latency requirements between all hosts so that certain global cost and performance metrics is optimized. The cost and performance metrics can be, for example, average structural latency between all communicating hosts in number of router hops, or sum of bandwidth between all pair of hosts and the distance between them in number of hops, or some combination of these two. This optimization problem is known to be NP-hard and heuristic based approaches are often used. The hosts in a system may vary in shape and sizes with respect to each other, which puts additional complexity in placing them in a 2D planar NoC topology, packing them optimally while leaving little whitespaces, and avoiding overlapping hosts.
There are several protocols by which components can connect to a network. As an example, there are several industry standards in existence such as Advanced eXtensible Interface (AXI), Peripheral Component Interconnect (PCI), and so on. In addition, several internal protocols have been developed for communication between components. In a complex system-on-chip, there are close to a hundred components all connected to the same network by which they communicate with memory. These components have evolved through different periods of time and through different architectural and performance preferences, due to which they chose to adopt different interface protocols. Components that expect to connect to each other over a NoC must convert now their communication into a language that is understood by each intended destination.
Packets are message transport units for intercommunication between various components. A NoC provides maximum benefit to a system when requests and responses are packetized. Use of packets allows a reduction in hardware cost so that no dedicated connections are required between components. Existing connections can be time shared for packets from different sources going to different destinations. If a section of the NoC is compromised, packets can be automatically re-routed to provide graceful degradation of the system instead of a deadlock. Hence, packetizing and bridging the chosen component protocol to a NoC provides a good interconnect solution.
Packetizing involves identifying a protocol that is flexible and compatible with many different protocols already in use. It should be able to work for all type of packets, reads, writes, barriers, posted or non-posted, ordered or out-of-order. It also should work for all packet lengths and sizes. Virtual channels inherent in the protocol should also be preserved over the NoC, or provided to the components as an enhancement.
To provide flexibility and adaptability, a bridging protocol employs wormhole routing, wherein the size of each flit is variable, and the number of flits are also variable. Additionally, the size of the flit, and hence the number of flits in the packet, may be different on the component size of the bridge than it is on the NoC side of the bridge.
FIG. 3 shows multiple fields used in a bridging protocol on an ingress side of a bridge, going from an originating component into the bridge. Such a bridge can be called an “originating bridge” or a “transmitting bridge”. As illustrated, the head flit includes 1 bit indicating the start of packet (Start-of-Packet bit), an X-bit destination node identification field, a Y-bit destination interface identification field, and a Z-bit virtual channel identification field. These fields present in the head flit allow the originating bridge and the routers of the NoC to choose the way through which the packet needs to be sent to its destination. No data is present in the head flit. Data payload provided by the component begins from the second flit. Width of the data packet may be variable and decided beforehand. Length, or the number of flits in the data packet, may be variable and decided on-the-fly by the SoC component/host. SoC component signals a tail flit by setting the End-of-Packet bit. Validity of the payload of the packet may be indicated by a valid bit. It is possible that a SoC component does not provide data on each clock cycle. Each valid flit of the packet may only be sent by the component if a credit is available. Initial credits available to the SoC component may be set up based on FIFO depths within the originating bridge. A credit is consumed each time the component sends a valid flit. A credit is released by the bridge each time a valid flit is received and processed. Credits are accumulated by the component as they are received from the bridge.
The simplicity and flexibility of this protocol allows any number of fields to be sent as the data payload, as long as they are decoded correctly by the destination component. The protocol is lightweight and for each data or metadata flit, adds only three extra bits: Valid, Start-of-Packet and End-of-Packet. For example, for server systems that require strict integrity of data, a CRC or parity field may be created by the component and added to the payload. The destination component on receiving this packet may strip the packet into the respective fields and match CRC or parity accordingly.
FIG. 4 shows the fields used in the bridging protocol on the egress side of the bridge, going into the destination component. This bridge is referred to as the “destination bridge” or “receiving bridge”. The data or metadata width of the destination bridge can be different from that of the originating bridge. Head flit can include 1 bit indicating the start of packet and the tail flit can include 1 bit indicating the end of packet. For a short packet of only 1 flit, both Start-of-Packet and End-of-Packet bits can be set at the same time. At the egress side of the bridge, all packet metadata such as destination node identification field, destination interface identification field and virtual channel identification fields are stripped away. All fields provided in the data payload by the originating component are preserved and presented to the destination component. At the destination component, each flit from the bridge to the component may be provided only in the presence of a credit. Initial credits within the bridge can be set up based on First in First Out (FIFO) depths within the destination SoC component. When a flit is sent to the destination component, a credit is consumed. As each flit is received and processed, the destination component releases a credit to the destination bridge. Bridges can also be configured to facilitate the conversion between different channel widths, such as converting flits for one channel into another flit for a channel with a different width.
Within a virtual channel, packets sent from an originating component A are received by a destination component B in the same order as they are sent. To preserve all traffic patterns as intended by the originating component, and to prevent deadlocks, the bridge and NoC may not do any reordering of packets.
Virtual channels are provided within a network to allow priority or isochronous packets to meet latency deadlines. To access a certain virtual channel, an originating component may choose a virtual channel and provide its information in the virtual channel identification field in the head flit of the packet. The originating bridge uses this information and uses a pre-decided routing table to arbitrate for this packet based on available credits, virtual channel priorities, and destination route. Virtual path from the source to the destination component can be maintained throughout the life of the packet within the NoC. Widths of the individual virtual channels are also flexible and may be programmed differently from each other as long as each width is less than the programmed width of the destination payload.
The destination bridge receives the packet with the virtual channel identification field intact, and then arbitrates the packet based on destination, virtual channel priorities, and destination component port. In this way, the virtual path and priority of a packet is maintained throughout the NoC, between the originating component and destination component.
The originating and destination bridges are flexible in terms of how they convert the packet on their egress sides. Each bridge may upsize and/or downsize the packet width to best suit the performance of the NoC. For instance, a protocol packet leaving the originating bridge and going further into the NoC may increase the size of the packet two-fold if the width of the NoC physical channel so allows, which as a result, reduces the latency seen within the NoC between origin and destination.
In the OSI (Open Systems Interconnection) model of computer networking, there are seven layers. Layer closest to the electrical interface is the Physical Layer, while the layer closest to software, the highest layer, is the Application Layer. Between the Physical Layer and the Application Layer, and in the same order, are the Data Link Layer, the Network Layer, the Transport Layer, the Session Layer and the Presentation Layer. The protocol described in this document sits between the Network and Transport Layers. FIG. 5 indicates the position of the NoC protocol. It is capable of converting any transaction in the layers below it, as well as a transaction from the Transport layer into the desired NoC packet protocol.